The accumulation of ice on aircraft proprotors, wings and other structural members in flight is a well known danger during low temperature conditions. As used herein, the terms "aircraft members" or "structural members" are intended to refer to any aircraft surface susceptible to icing during flight, including proprotors, wings, stabilizers, engine inlets and the like. Attempts have been made since the earliest days of flight to overcome the problem of ice accumulation. While a variety of techniques have been proposed for removing ice from aircraft before or during flight, many prior systems or techniques experience various drawbacks or possess certain limitations.
One approach to ice management that has been used is so-called thermal de-icing. In thermal de-icing, the leading edges, that is, the portions of the aircraft that meet and break the airstream impinging on the aircraft, are heated to prevent the formation of ice or to loosen accumulated ice. The loosened ice is then blown from the structural members by the airstream passing over the aircraft.
In one form of thermal de-icing, heating is accomplished by placing an electrothermal pad, including heating elements, over the leading edges of the aircraft, or by incorporating the heating elements into the structural members of the aircraft. Electrical energy for each heating element is typically derived from a generating source driven by one or more of the aircraft engines or transmissions. The electrical energy is intermittently or continuously supplied to provide heat sufficient to prevent the formation of ice or to loosen accumulating ice.
With some commonly employed thermal de-icers, the heating elements are configured as ribbons, e.g. interconnected conductive segments, that are mounted on a flexible backing. The conductive segments are separated from each other by gaps, e.g. intersegmental gaps, and each ribbon is electrically energized by a pair of contact strips. When applied to a wing or other airfoil surface, the segments are arranged in strips or zones extending spanwise or chordwise of the aircraft wing, rotor or airfoil. One of these strips, known as a spanwise parting strip, is disposed along a spanwise axis which commonly coincides with a stagnation line that develops during flight in which icing is encountered. Other strips, known as chordwise parting strips, are disposed at the ends of the spanwise parting strip and are aligned along chordwise axes. Other zones, known as spanwise shedding zones, are typically positioned above and below the spanwise parting strip at a location intermediate the chordwise parting strips. Between adjacent zones, a gap, known as an interheater gap, sometimes exists.
One known method for de-icing causes electrical current to be transmitted continuously through parting strips so that the strips are heated continuously to a temperature above 32.degree. F. In the spanwise shedding zones, on the other hand, current is transmitted intermittently so that the spanwise shedding zones are heated intermittently to a temperature above about 32.degree. F.
While this technique of heating the various zones generally is effective to melt ice (or prevent its formation) without the consumption of excessive current, a problem exists in that melting of ice in the inter-segmental and interheater gaps can be difficult or impossible. Moreover melting of ice on or around the contact strips can also be difficult or impossible. Accumulation of ice in the gaps and on the contact stripe is particularly undesirable because the unmelted ice can serve as "anchors" for ice that would be melted but for the ice accumulated in the gaps or on the contact strips.
Another problem with prior thermal-based systems is their lack of reliability. Aircraft members, such as rotors of a helicopter or proprotors of tiltrotor aircraft, undergo much strain and stress associated with aircraft operation. Ongoing use of aircraft inevitably results in some damage to aircraft components. With respect to heating elements integrated within an aircraft member, breaks in blanket circuitry can cause thermal de-icing systems to fail, posing serious risk to aircraft crew and equipment during cold weather operations. And yet another concern with heating element circuitry is the potential for inconsistency, e.g. hot spot or cold spot generation, and larger than acceptable power consumption.
Problems may also be encountered where strips are run along the entire length of the aircraft. The size of the ice being shed by the aircraft member can cause a hazard to the aircraft's fuselage. If the particle of ice is too large, it could hit and may even penetrate the fuselage.